Many-Body Effects and Photo-Induced Spatially Condensed Phases in the Exciton Systems in Layered Crystals

我们制备了高质量的BiI_3、GaSe和PbI_2层状晶体。在这些晶体中，我们研究了高密度激子的空间行为与多体效应和一些新的光致凝聚相如玻色-爱因斯坦凝聚（BEC）的关系。在BiI_3中，在层错界面的准二维平面缺陷处出现了称为层错激子（SFE）的激子态。利用空间分辨泵浦探测光谱和空间分辨光致发光谱研究了N_2激光泵浦染料激光器激发的超临界流体的空间行为。从这些光谱中，证实了高密度激子在二维空间中集体相干运动的存在。我们用锁模Nd：YAG激光系统在皮秒时间范围内研究了这些光谱。事实证明，激子的分量以与极化激元模式的群速度相比的速度快速膨胀。对于这个系统以及GaSe激子，我们还应用了一种新的技术的简并四波混频（DFWM）与空间隔离的两个激光束调谐在激子的共振能量。由于激子的传播，在两个激发光斑的中点处首先观察到DFWM信号。将观测到的高密度单粒子效应的反常行为与基于非线性薛定谔方程的理论计算进行了比较，并讨论了与激子玻色-爱因斯坦凝聚的关系。

We'v prepared good quality layered crystals BiI_3, GaSe, and PbI_2 for this project. In these crystals we have studied spatial behavior of the excitons at high density in connection with many-body effects and some new photo-induced condensed phases such as the Bose-Einstain condensation (BEC).In BiI_3, exciton states called the stacking fault excitons (SFE) appear at a quasi two-dimensional planar defect of a stacking fault interface. The spatial behavior of SFE excited with a N_2-laser-pumped dye-laser was investigated by detecting space-resolved pump-and-probe and space-resolved photoluminescence spectra. From these spectra, there is confirmed the existence of the high density excitons moving collectively and coherently in the two-dimensional space. We examined these spectra in the picosecond time region with a mode-locked Nd : YAG laser system. It turns out that a component of the excitons expands rapidly with a velocity compared to the group velocity of the polariton mode. For this system as well as GaSe excitons, we also applied a new technique of the degenerate four-wave-mixing (DFWM) with spatially isolated two laser beams tuned at the resonant energies of the excitons. The DFWM signals was observed firstly at the middle point between the two exciting laser spots due to the propagation of the excitons. The observed extraordinary behavior of the SEE at high density is compared with the calculation based on the theory composed of a nonlinear Schrodinger equation, and the connection with the Bose-Einstain condensation of excitons is discused.